The present exemplary embodiment relates generally to the printing arts. It finds particular application in conjunction with a printing system which extends the color gamut available with process colors using multi-pass printing. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
The color demands of images to be rendered by a marking device are usually specified in a device independent color space as part of a page description language (PDL). Color spaces typically used in PDL files include RGB, CMYK, L*a*b*, and PANTONE. L*, a*, b*, for example, are the independent space representations of the CIE (Commission Internationale de L'eclairage) for color standards which are often utilized in the functional modeling of these color demands. L* defines lightness, a* corresponds to the red/green value and b* denotes the amount of yellow/blue. The PDL source color representations are transformed into representations which the device can reproduce with available colorants, such as cyan, magenta, yellow and black (CMYK) representations.
A look up table (LUT) is used to determine which combination of available colorants (typically cyan, magenta, yellow, and black) will yield the desired colors specified. The look up table may be derived by printing test patches and examining the output. The input space (domain) describes all the possible ways of combining the three primary colorants (for example, cyan, magenta, and yellow but many other triplets are possible). Typically, the halftone density of a colorant is specified by an 8 bit integer (a whole number between 0 and 255 in base 10 notation). 255 corresponds to the maximum density which can be achieved. The domain of the transformation consists of a three dimensional cube, 255 units on a side with one corner at the origin. The co-domain space of the transformation is the color gamut of the marking device, the three dimensional volume that indicates all the L*, a*, b* values which are accessible by combining the three (or four) colorants.
Different color devices have different color capabilities. Every color device, whether it is a color scanner, a color marking device, or a color display monitor, has a color gamut, i.e., a range of colors that it can capture, produce, or display. While most color display monitors can display hundreds of thousands of colors using gray scale or continuous tones (contone), color marking devices usually have a significantly smaller number of producible colors. In halftone printing, for example, the image is made up of an array of pixels. The chromaticity of a given area is increased by turning on an increasing number of pixels of the colorant. The maximum density of a colorant is achieved, for a given area, when 100% of the pixels are turned on. A pixel is the smallest element of a printing system that can be independently controlled.
Various attempts have been made to expand the color gamut of marking devices, either to allow a closer match with the rendering of an image on a color display monitor, or to produce colors to meet specific customer requests (“custom colors”). Often the colors which tend to be outside the color gamut are those which have a high intensity of two or more of the colorants. In inkjet printing systems and offset lithography, “spot color” or “high-fidelity” color printing has been developed in which conventional cyan, magenta, yellow and black (CMYK) inks are augmented with additional primary colors beyond the usual four primary colors used to produce the process color output. These additional inks are used for extending the color gamut of the process color output (high fidelity color), and thereby more closely emulate standardized spot colors, such as those defined by Pantone. However, additional hardware is needed in the form of printing units.
In a typical xerographic marking engine, such as a copier or printer, a photoconductive insulating member is charged to a uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member, which corresponds to the image areas contained within the document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a developing material. Generally, the developing material comprises toner particles adhering triboelectrically to carrier granules. The developed image is subsequently transferred to a print medium, such as a sheet of paper. The fusing of the toner onto the paper is generally accomplished by applying heat to the toner with a heated roller and application of pressure.
Adding an additional colorant in a xerographic marking device poses design problems since the additional equipment includes not only an additional developer housing for the fifth colorant, but also an additional charging corotron and an exposure station.